Universal ion-selective meter

ABSTRACT

The proposed multipurpose ion-selective sensor is designed to be operational in any spatial position in harsh environment and incorporates a housing tube ( 1 ) where a number of elements are fixed and electrically connected to a measurement transmitter; these members include: a detachable indicator electrode ( 3 ) which employs a solid indicator system ( 33 ) with a solid membrane contacting the analysed liquid; a detachable reference electrode ( 4 ) with a hollow body ( 20 ) accommodates a potential-forming semi-element ( 21 ), a connecting member ( 22 ) and electrolyte ( 29 ) filled with a solid dispersed material forming a spatial structure ( 30 ) in the electrolyte ( 29 ), whereby the structure is rigidly linked with internal surface ( 31 ) of the reference electrode ( 4 ) body ( 20 ), the potential-forming semi-element ( 21 ) and with the connecting member ( 22 ).

APPLICATION AREA

The invention relates to devices for ion composition and redox potentialmonitoring in process fluids, natural water and sewage, andspecifically, a multipurpose ion-selective sensor.

BACKGROUND OF THE INVENTION

Numerous ion-selective sensors designed for monitoring of hydrogen ionsin various process fluids are widely known.

Such examples include, in particular, a ion-selective sensor describedby P. Meier, A. Lohrum and J. Gareess (Editors) in the study, “Practiceand Theory of pH Measurement”, Ingold Messtechnik A G, CH-B902Undorf/Switzerland, 1989, p. 15. This combined sensor comprises a glasstube body incorporating glass indicator and reference electrodes filledwith a liquid electrolyte.

Another known ion-selective sensor (a Switzerland's Ingold booklet“pH/Redox Measurement in Biotechnology. pH-Electrodes. IndustrialProbes. Sensors,” Ingold Messtechnik A G, CH-8902 Urdorf/Switzerland1990, p. 10) has a similar design, but its reference electrode is filledwith a gel-type electrolyte.

The both above mentioned sensors have a monolithic glass design andrequire refillings of their reference electrodes. These sensors are onlysuitable for predefined spatial positions and are inoperable in harshconditions (elevated temperatures and those below 0° C., high pressures,zero-gravity and other conditions). Sensors intended for redox potentialmonitoring are widely employed at present. For instance, a sensor with areference electrode filled with gel electrolyte and an indicatorelectrode with a platinum or gold sensing element have been described ina booklet titled “PH/Redox Measurement in Chemical Processes.pH-Electrodes. Industrial Probes. Sensors,” published by IngoldMesstechnik A G, CH-8902 Urdorf/Switzerland, 1990, p. 16.

Similarly to the above mentioned ion-selective sensors, this devicecomprises an integral, monolithic glass design which is only operationalin a predefined spatial position and is not intended for harshconditions.

Another known ion-selective sensor (the US' Foxboro Company booklet,“pH/ORP/ISE Sensors B71 pH Series,” The Foxboro Company, Foxboro Mass.02035-2099, 1993, p. 1) was designed for fluorine and hydrogen ionactivity monitoring and is suitable for redox potential monitoringapplications.

This sensor incorporates a plastic housing tube where a detachableindicator electrode is rigidly fixed in addition to a referenceelectrode that has a hollow body filled with liquid or gel electrolytecontaining a solid dispersed material which forms a spatial structurethat is freely movable inside the reference electrode. An indicatorelectrode filled with liquid electrolyte, together with a solidion-selective sensor, are installed into the housing tube for ionactivity monitoring, and an indicator electrode with a platinum sensingelement is inserted into the housing tube for redox potentialmonitoring.

Compared to the previously mentioned sensors, the claimed device is moreflexible in use and therefore has wider applications.

However, all above mentioned disadvantages apply to the latter sensor.They are attributed to the liquid or gel-type electrolyte present in thereference electrode and the liquid electrolyte inside the indicatorelectrode. This requires frequent refillings of the reference electrodeelectrolyte, poses spatial limitations for sensor operation andrestricts harsh-environment applications.

DETAILED DESCRIPTION OF THE SPECIFIED EMBODIMENTS

The proposed invention is built around the design of a multipurposeion-selective sensor in an embodiment that would eliminate electrolytemovements inside the electrodes and its entry into the analysed liquidthereby making it functional in any spatial position, critical operatingenvironments, and under any ion composition and redox potential.

This challenge has been met by a multipurpose ion-selective sensor whichincorporates a housing tube with a detachable indicator electrode thatis fixed to, and is electrically connected with a measurementtransmitter, whereby the electrode has an indicator system with asensing membrane immersed into the analysed liquid; the referenceelectrode comprises a hollow body with electrolyte filled with a soliddispersed material forming a spatial structure in the electrolyte, apotential-forming semi-element and a connecting member which iscontacting the analysed liquid. According to the proposed layout, boththe indicator system and its sensing probe have a solid-state design;the reference electrode has an internal surface, and its spatialstructure is rigidly linked with this surface and the connecting member.

In this way, the proposed ion-selective sensor employs electrodes whichare essentially immune to gravity and enable any spatial position fornormal operation. This design eliminates electrode damage at hightemperatures (around 200° C.) high pressures (around 5 MPa),zero-gravity conditions and low temperatures of some −60° C., therebyensuring full operability in extreme conditions. Moreover, the designeliminates the electrolyte entry from the reference electrode and thusgreatly extends the useful service life of the claimed sensor. Thissensor design enables its application for redox potential measurementsin analysed liquids with no regard to sensor spatial position andoperating environment.

One and two-chamber designs are available for the reference electrode.For the both embodiments, the electrolyte spatial structure in eachchamber of the reference electrode is rigidly linked with internal wallsof said chambers, the potential-forming member, each connection memberand external wall of the internal chamber.

In the single-chamber reference electrode embodiment, the said rigidlink of the spatial structure is achieved by a solid dispersed materialof the reference electrode electrolyte, in quantity sufficient for 5-30wt. % concentrations whereby the said material contains acrylamide andNN¹-methylemebisacrylamide monomers in proportions of 5-35 weight partsof acrylamide per one weight part of NN¹-methylemebisacrylamide as wellas a monomer polymerisation initiator; in the two-chamber referenceelectrode embodiment, the second solid dispersed material is present inthe internal electrolyte, in quantities suitable for 0.1-5 wt. %concentrations and with a similar composition to the solid dispersedmaterial of the electrolyte contained in the reference electrode body.

This enables various designs of solid-electrolyte indicator electrodesto be mounted in the sensor housing tube where the said electrodesshould be the most appropriate for individual analysed liquids and, mostimportantly, will enable the solid-state configuration of theirindicator system and the sensing member.

For instance, the indicator electrode can be configured with a hollowglass body with its one end portion incorporating an indicator systemcomprising an envelope made of sensitive glass, and an internal surfacewhere the solid electrolyte is connected with a measurement transmitterby an electric conductor. The said sensitive glass can be either aion-selective grade or a glass sensitive to electron transfer into redoxprocesses.

An indicator electrode can be installed for individual liquids underanalysis where the electrode can be either selective to a specified ionof the analysed liquid or, optionally, be suitable for monitoring theredox potential.

For example, the indicator electrode can comprise an inert material bodywith its indicator system located at one end portion, the said systemcomprising a ion-selective membrane having an internal surface with asolid electrolyte thereon and connected to the measurement transmitterby an electric conductor; alternatively, an indicator electrode can bemade with an inert material body and its indicator system provided inone end portion, with the said system comprising a noble-metal sensitivemembrane connected to the measurement transmitter by an electricconductor.

The sealed sensor housing tube design and reliability improvements aswell as easier sensor maintenance, assembly and disassembly forelectrode replacement are achieved by means of the first sensor sealingmember enabling the detachable fixing of electrodes within the housingtube; the first sealing member can accommodate at least one sealingelement made of an elastic O-ring positioned on the external surface ofa side wall of each electrode; the sensor housing tube can be madehollow with removable inert-material filler therein, fixed in thehousing by the second sealing member; holes are made in the filler inthe latter case which are essentially parallel to the housing tube'slongitudinal axis, and the electrodes are rigidly fixed in these holeswhere each filler comprises a cylinder member with external surfacefitting the internal surface of the sensor housing tube, while thesecond seal having at least one sealing element comprising an elasticO-ring securely positioned on the external surface of the filler sidewall.

Low-volume monitoring of analysed liquids including laboratorymeasurements can be facilitated by an embodiment where the sensorhousing tube can optionally comprise the first and the second coaxiallyfixed, detachable members which are isolated from each other; thereference electrode is rigidly positioned in the first portion of thehousing tube and at least one indicator electrode is rigidly fixed inits second portion.

The sensor maintenance is facilitated by electrode connections with themeasurement transmitter by means of current terminal wires connected attheir one end by detachable contact sockets to wires of relatedelectrodes and at the other end to a cable connector of the measurementtransmitter.

For measurement accuracy improvements, the sensor can be fitted with aRTD which is electrically connected to the measurement transmitter, theRTD can be located outside the sensor in proximity of the electrode areaor inside the sensor.

For convenience and operating simplicity, the sensor housing tube can befitted with a holder for securing the sensor in the analysed liquidzone; special grooves should be made in the sensor housing tube in theholder fixing portion, said grooves should be provided with elasticO-rings and the holder should be connected with the housing by means ofa threaded connection and be sealed by the mentioned elastic members.Besides, the sensor can be fitted with at least one intermediate memberwhich is hermetically sealed between the sensor housing and the holder.

As such, the claimed design enables a multipurpose sensor to be made formonitoring ion activity of various chemical elements including hydrogenions, and redox potential by means of the same device which is operablein any spatial position and extreme conditions; the said deviceeliminates reference electrode refillings with electrolyte therebysignificantly extending the useful service life of the sensor and, asnecessary, enables quick and easy replacements of electrodes secured inthe sensor housing tube.

BRIEF DESCRIPTION OF THE DRAWINGS

For better understanding of the claimed fundamentals, individualapplication examples are hereinafter provided with references toattached drawings including:

FIG. 1—A multipurpose ion-selective sensor made according to the presentinvention, a longitudinal section;

FIG. 2—The reference electrode made according to the present invention,a longitudinal section;

FIG. 3—The multipurpose ion-selective sensor fabricated in compliancewith these claims; alternative embodiment, a longitudinal section with atorn-off segment;

FIG. 4—Cross-section IV—IV in FIG. 3;

FIG. 5—Cross-section V—V in FIG. 3;

FIG. 6—Multipurpose ion-selective sensor made in accordance with thepresent invention; alternative embodiment, a longitudinal section.

THE BEST EMBODIMENT OF THE INVENTION

The multifunctional ion-selective sensor made according to the presentinvention incorporates the housing tube (FIG. 1) which comprises a metalcylinder that has two holes (2) made essentially parallel to thelongitudinal axis of the said cylinder; the holes accommodate anindicator electrode (3) and a reference electrode (4). One, two and moreholes can be optionally made in the housing tube for placing theindicator electrodes. The number of indicating electrodes depends oncomposition of analysed liquid, that is on availability of various ionswith activities which can be simultaneously monitored by the proposedsensor.

Electrodes (3) and (4) have cross-sections which are smaller than thoseof related holes (2), to enable fast removal of electrodes (3) and (4)from the housing tube (1) when necessary. The electrodes (3, 4) aresealed in the housing (1) by sealing members made of elastic O-rings(5), such as rubber rings, and placed on the external side wall surface(6, 6 ¹) of electrodes (3, 4). Each electrode is fixed in the housingtube (1) for protection from longitudinal motion by means of arestrictor (7) comprising a collar which is rigidly fixed (pressed) inthe sensor housing tube (1).

Each electrode (3, 4) is electrically connected by a wire (8, 8 ¹) tothe measurement transmitter (not shown in the drawing). This electricconnection comprises current terminal wires (9, 9 ¹) connected at oneend with detachable contact sockets (10, 10 ¹) to conductors (8, 8 ¹) ofrelated electrodes (3, 4) and at another end to a socket (11) of themeasurement transmitter cable connector.

The sensor has a hollow holder (12) for positioning it in the analysedliquid zone. A tubular intermediate member (13) is installed between thehousing tube (1) and the holder (12) for extension of the housing (1),where the said member (13) is tightly fixed on the housing tube (1) andthe holder (12), with the latter having a mounting nut (14). Annulargrooves (15) are made on the external surfaces of the housing tube (1)and the intermediate member (13), the said grooves have elastic O-rings(16) and a thread connector (17). Depending on size of a process vesselfor the analysed liquid and a sensor mounting location (not shown in thedrawing), the sensor can optionally have two, three or more intermediatemembers (13) with different lengths, or no such members at all. In thisway, the sensor housing tube (1), the holder (12) and the intermediatemember (13) comprise an encapsulated, collapsible assembly whicheliminates entry of the analysed liquid therein, whatever the externalconditions exist.

The sensor is packaged with a RTD (18) for measurement of analysedliquid temperature, the RTD is positioned inside the said encapsulated,collapsible assembly which is contacting the internal surface of thesensor housing tube (1) and is connected to the measurement transmitterby electric conductors (19). Optionally, the RTD (18) can be mounted onexternal surface of the sensor (not shown in the drawing) in proximityof the electrode (3, 4) zone.

As described above, the electrodes (3, 4) are retractably mounted in thehousing tube (1) and can be quickly replaced as required.

FIG. 1 illustrates the reference electrode (4) which comprises a hollowglass body (20) with a Ag/AgCl potential-forming semielement (21)connected by the electric wire (8), and a connecting member (22) isavailable for contact with the analysed liquid. In this configuration,the connecting member (22) comprises a hole made in the end portion (23)of the reference electrode (4) projected from the housing (1), with thesaid end portion incorporating a porous ceramic membrane (24). Inanother possible connecting member configuration, the membrane can bemade of a porous glass or asbestos cord.

A rubber plug (26) and a cap (27) are rigidly mounted in the body (20)of the reference electrode (4) near its end (25), opposite theprojecting end (23), for securing the electric conductor (8); they areisolating the body (20) space (28) of the reference electrode (4). Thesaid space (28) is filled with electrolyte (29) containing a soliddispersed material which forms a spatial structure (30) in theelectrolyte where this structure is rigidly linked with internal surface(31) of the reference electrode (4) body (20), the potential-formingsemielement (21) and the connecting member (22). Optionally, the saidelectrolyte can be based on a saturated potassium chloride solution.

This solid dispersed electrolyte contains acrylamide andNN¹-methylemebisacrylamide monomers in the following proportion: from 5to 35 weight parts of acrylamide per one weight part ofNN¹-methylemebisacrylamide, in addition to any known monomerpolymerisation initiator such as riboflavin.

The solid dispersed material is present in the electrolyte (29) inquantities sufficient for its concentrations from 5 to 30 wt. %.

Employment of the solid dispersed material with the proposed compositionand concentration range in the electrolyte (29) will facilitate build-upof an elastic polymer spatial structure within the electrolyte (29)featuring rigid links with the internal surface (31), thepotential-forming semi-element (21) and the connecting member (22). Theelectrolyte (29) comprises a rubber-type material that can withstand theenvironmental impacts in extreme conditions and maintain a fixedposition within the interior (26) of the reference electrode (4) body(20) in any spatial arrangement of the sensor. The mentioned monomerratio will secure strength of mechanical links with the describedreference electrode (4) members within the spatial structure (30).

When more than 35:1 acrylamide is present in the system, the soliddispersed material is likely to fail forming the aforenamed spatialstructure (30), but rather the long fibres with irregularinterconnections.

If less than 5:1 acrylamide is present, this is likely to lead to aheterogeneous spatial structure in the electrolyte (29) with brittlemechanical properties and weak links with body walls (20) and with theconnecting member (22).

When less than 5 wt. % of the solid dispersed material is present in theelectrolyte (29), rigid connection of the spatial structure (30) withthe connector member (22) can be affected by environmental impacts underextreme conditions.

If more than 30 wt. % of the solid dispersed material is present in theelectrolyte (29), salt solubility would be lower and would hamper themeasurement accuracy.

The indicator electrode (3) of the proposed sensor can employ any knownindicator electrode design featuring a solid-state, ion-selectivemembrane with a solid electrolyte, or any known indicator electrode witha solid member which is sensitive to electron transfer into redoxprocesses.

FIG. 1 highlights the indicator electrode (3) with a hollow glass body(32), whereby an indicator system (33) is positioned and is extendingfrom the body (32) at one end portion, the said system includes asensing member in the form of a spherical envelope (33 ¹) made ofion-selective glass, for example the grade selectively sensitive tohydrogen, sodium ions or those of any other element.

The said envelope (33 ¹) has internal surface on which a solidelectrolyte (34) is positioned and is connected to the measurementtransmitter by electric wire (8).

The similar design applies to an indicator electrode with a sensingmember in the form of a glass envelope sensitive to electron transfer toredox processes.

In these embodiments of the indicator electrode, the sensor employs adevice (35) for protection of the envelope (33 ¹) from mechanicaldamage, where the said device could comprise a slotted collar fixed onthe housing tube (1) of the sensor by a threaded connector (36) in theenvelope (33 ¹) zone.

The indicator electrode can have any other known embodiment that fitssimilar applications, but it is critical that the indicator system andsensing member be the solid-state design.

For instance, in one configuration, the indicator electrode (not shownin the drawing) can have a body made of inert material such aspolyarylate, with an indicator system at its one end portion, where thesensing member of the said system comprises a solid ion-selectivemembrane with internal surface and a solid electrolyte thereon,connected to the measurement transmitter by electric wire, oralternatively the indicator electrode can comprise an inert materialbody such as glass, with its indicator system mounted at its one endportion, comprising a sensitive membrane made of a noble metal andconnected to the measurement transmitter by electric wire.

FIG. 2 illustrates another reference electrode (37) embodiment where ahollow glass body (38) has a fixed hollow glass envelope (39)incorporating a potential-forming semi-element (21) and internalelectrolyte (40) with a solid dispersed material forming a spatialstructure (41) in the said electrolyte (40). At one end portion, near aconnecting member (22) of the body (38), the envelope (39) has aconnecting member (42) for contact with electrolyte (29) within thereference electrode (37) of the body (38) where the said member can bemade similar to a connecting member (22) of this body (38); there is arubber or epoxy plug (43) for sealing the envelope (39) area (44) at theother end portion.

The spatial structure (41) is rigidly linked with the internal surfaceof the envelope (39), with the potential-forming semi-element (21) andthe connecting member (42), while the external surface of the envelope(39) is rigidly linked with the spatial structure (30) of electrolyte(29) present in the interior (45) of the reference electrode (37) body(38), where the said interior (45) is also rigidly linked with internalsurface (31 ¹) of its body (38) and its connecting member (22).

The solid dispersed material is present in the internal electrolyte (40)in quantity sufficient for 0.1-5 wt. % concentrations and has a similarcomposition to that of a solid dispersed material in electrolyte (29)present in the body (38) of the reference electrode (37). The abovementioned concentrations of the solid dispersed material are sufficientfor building a spatial structure (41) which is rigidly linked with thesaid envelope (39) members. As soon as there is no contact between theinternal electrolyte (40) and the analysed liquid, the former is immuneto environmental impacts and thereby enables the appropriateconcentrations of the solid dispersed material for required measurementaccuracy.

No spatial structure (41) can be formed if the solid dispersed materialconcentration in the electrolyte (40) is lower than 0.1 wt. %, andmeasurement accuracy can be hampered when its concentration is above the5 wt. % level.

FIGS. 3 through 5 illustrate a sensor with a hollow metal housing tube(45) which accommodates an insert (46) made of inert material such asheat-resistant plastic. Through holes are made in the insert (46)essentially parallel to a longitudinal axis of the housing tube (45), toaccommodate electrodes (47, 3, 48) rigidly fixed therein by means offilling a silicone hermetic glue or epoxy resin, as an example.

The sensor has one reference electrode (48) which is similar to thatshown in FIG. 2, and two indicator electrodes (3) and (47). Oneindicator electrode (3) has a similar design to that shown in FIG. 1 andis intended for monitoring hydrogen ions, and another indicatorelectrode (47) features a spherical glass sensing member (49) which issensitive to electron transfer to redox reactions; the latter has asolid electrolyte (50) thereon which is connected by electric wire (8)to the measurement transmitter.

The insert (46) comprises a cylindrical member with its outer surfacematching the internal surface of the housing tube (45), wherein the saidinsert (46) is designed for easy removal from the housing (45). Theinsert (46) is fixed in the housing tube (45) by a sealing devicecontaining O-rings (51) made of elastic material such as rubber andplaced in annular grooves (52) made on the external side surface of theinsert (46). The number of these grooves (52) and O-rings (51) dependson aggressiveness of the analysed liquid and extreme conditions expectedfor sensor operation.

Depending on operating conditions and measurement accuracy requirements,the reference electrode (48) of this sensor can be optionally aone-chamber (FIG. 1) or a two-chamber (FIG. 2) design.

The insert (46) has another opening which is essentially parallel to theelectrode holes (3, 47, 48), the said opening accommodates a heatconductor (53) with its one end projecting from the housing tube (45)and contacting the analysed liquid and another end contacting the RTDdevice (54).

FIG. 6 shows a sensor configuration with its housing tube (55) made ofinert material such as polystyrene, and comprises two parts (56) and(57) which are coaxially fixed to each other and enable quickdisconnection; they are isolated from each other by means of aconnecting member (see FIG. 6). In the first portion (56) of the sensorhousing tube (55) a reference electrode (59) is rigidly fixed, itstwo-chamber design is similar to the reference electrode (37) shown inFIG. 2, but the body of the former is made of inert material andcomprises the first part (56) of the sensor housing tube (55); theconnecting member (22) is contacting the analysed liquid and ispositioned on the side surface of the housing (55) first part (56), inproximity of its connection with the second part (57). The second part(57) of the sensor housing tube (55) accommodates a rigidly fixedindicator electrode (60) which can be optionally made similar to theabove mentioned one, when its body is made of inert material. In onemodification, two, three or more indicator electrodes (60) can berigidly fixed in this part (57) of the housing (55).

The connection member between two parts (56) and (57) of the sensorhousing tube (55) is fitted with a shaped collar (61) made of inertmaterial such as plastic, pressed in the first part (56) and effectivelyisolating its interior. A recess (62) is made in the collar (61) foraccommodation of an indicator electrode (60) portion with its associatedelectric wire (63) as well as a through hole for a contact socket (10)and a plug (64) for isolation of the recess (62) and opening fromelectrolyte in the body (56) of the reference electrode (59). In itsportion projecting from the first portion (56) of the housing tube (55),the collar (61) of the connecting member (59) has external thread (65)and a sealing member comprising optionally a rubber O-ring, while thesecond part (57) of the housing (55) is rigidly fixed thereon by meansof the said thread (65) and is sealed relatively to the first part (56)by a rubber O-ring. The electric conductor wire (63) of the indicatorelectrode (60) is connected to the contact socket (10) switched to themeasurement transmitter cable. The reference electrode (59) has a rubberand a plastic O-rings (67, 67 ¹ and 67 ¹¹) through which the currentterminal wires (68) and (68 ¹) of the indicator electrode (60) andreference electrode (59) are passed for connection of electric wires(63) and (8) of electrodes (60) and (59) with a measurement transmittercable socket (11).

Table 1 hereinafter presents five examples of reference electrodeelectrolyte compositions.

TABLE 1 Sensor design Total monomer concen- drawing, tration, wt. %according in reference A *:M ** monomer ratio Salt concentration,mole/liter to the electrode in reference in reference present body elec-in internal electrode body in internal electrode body in internal No.invention trolyte electrolyte electrolyte electrolyte electrolyteelectrolyte Details 1 FIG. 1  5 — 35:1 — 3M KCl — Ratings: temperaturerange of −60° C. to 200° C.; 1.6 MPa pres- sure 2 FIG. 1 18 — 20:1 — 3MKCl — Same 3 FIGS. 3 30 5   35:1  5:1 1M KCl 3M KCl Same through 5 4FIG. 6 10 0.1 20:1 10:1 2M KCl 1M KNO₃ Ratings: temperature range of−60° C. to 80° C.; 1.6 MPa pres- sure 5 FIG. 6 10 2.5 10:1 10:1 0.1M KCl5M KNO₃ Same * Acrylamide. ** NN¹-methylenebisacrylamide.

It can be seen from the five examples above that each sensor produced inaccordance with the present invention is operable at extreme conditions.

The multipurpose ion-selective sensor has the following operatingprinciple:

The proposed sensor is immersed into the analysed liquid in its portionwhere the indicator system (33), a sensing member of the indicatorelectrode (3, 47, 60) and a connecting member of the reference electrode(4, 37, 48 and 59) are mounted. The resultant solid-state indicatorelectrode (3,47 and 60) potential is proportional to activity logarithmof the analysed ion. The reference electrode (4, 37, 48 and 59)generates the potential which is not dependent on analysed liquidcomposition. Transfer of salt ions from the reference electrode (4, 37,48 and 59) electrolyte (29, 40) that is saturated by this salt closesthe electric circuit through the analysed liquid whereby the potentialdrop is directly proportional to an analysed ion activity logarithm.When it is necessary to change the function of the indicator electrode(32, 47, 60), the existing indicator electrode is replaced by anotherone; this enables measurements of a different parameter, for instance,ion activity of hydrogen or any other chemical. In case of failure ofany sensor electrode (3,47,60,4,37,48 or 59), sensor operation can beresumed after easy and simple replacement of a faulty electrode by a newone.

In another embodiment (Example 6), the multipurpose ion-selective sensorcan be manufactured complete with an interchangeable solid-electrolyteindicator electrode and a reference electrode shown in FIG. 1. Asolid-electrolyte glass electrode is used as an indicator electrodeshown in FIG. 1; the said electrode under-goes thermal water and steamsterilisation. The indicator system of the pH-metering indicatorelectrode comprises a 9 mm diameter sphere welded into a glass tube with6 mm external diameter. The total length of the indicator electrode,including a pin plug, is 92 mm. The reference electrode comprises aglass body with 5 mm diameter and 82 mm length (together with a pinplug). The lower end portion of the reference electrode has a weldedconnecting member made of a porous ceramic material. The Ag/AgClpotential-forming semi-element is immersed into a thermally resistantelectrolyte based on saturated solution of potassium chloride and asolid dispersed polymer containing acrylamide andNN¹-methylenebisacrylamide with 30 wt. % total concentration and 10:1ratio. A pin plug terminal made in the glass body is sealed by apermanent-action glue.

The sensor made according to the present invention is installed in afermenter with a 100 liter volume by means of a 25 mm dia. weld-insocket in a fermenter bottom in a reversed position and is fixed by acaptive nut. Another socket with a 15° canting angle is used for animmersion sensor, InFit 764, fitted with a combined sterilised pHelectrode, 465-5-S7 (both the electrode and the sensor are fromSwitzerland's Ingold). A similar Ingold sensor is installed in ahorizontal weld-in socket.

Three above mentioned sensors should be sterilised in a nutrient mediumdirectly in the fermenter, followed by growing of Pen. Chrisogenum. Acomplex nutrient medium is used. Automated pH monitoring uses all abovementioned sensors connected to related measurement transmitters.Cultural medium samples are regularly taken from the fermenter for pHmeasurements by a bench pH-meter. The analytical laboratory results arethen compared with Ingold sensor readings and those produced by theclaimed sensor.

The Ingold sensor is installed horizontally and cannot provide thefermenter process pH measurements a soon as the indicator electrodeelectrolyte cannot provide, in this position, an electrical contactbetween the internal surface of its indicator system and thepotential-forming semi-element, and the electrolyte contact is lost inthe reference electrode between the potential-forming semi-element andthe connecting member. Both the claimed sensor and that installed with a15° canting angle provide readings which closely correlate with thelaboratory electrode indications.

Test results are summarised in Table 2.

TABLE 2 pH readings 465 SK2403S Time, claimed model bench DifferentialpH readings No. h sensor sensor device (4)-(5) (3)-(5) (3)-(4) 1 2 3 4 56 7 8 1  0 6.22 6.17 6.20 −0.03  0.02  0.05 2 24 6.32 6.33 6.32 0.010.00  0.01 3 48 6.84 6.89 6.84 0.05 0.00 −0.05 4 72 6.86 6.87 6.86 0.010.00 −0.01 5 96 6.24 6.24 6.28 −0.04  −0.04   0.00 6 120  6.31 6.35 6.340.01 −0.03  −0.04

It can be seen from the examples above that the pH measurements producedduring a long fermentation process by the claimed sensor installed in areversed position have been found to be in good agreement with readingsof a commercial Ingold sensor installed at a 15° canting angle and theRADIOMETER laboratory electrode. The maximum deviation of readings iswithin 0.05 pH. However, the Ingold sensor appears inoperable wheninstalled horizontally.

Measurement accuracy evaluation for the claimed sensor is performedfollowing multiple sterilisations and a mounting scheme, together withthe Ingold's sensor, which is similar to the pH monitoring experimentsdescribed above. The claimed sensor is mounted in the fermenter in areversed position while the Ingold's one at the 15° canting angle. Thefermenter is sterilised under 132° C. temperature during one hour. Theconsecutive sterilisation procedures are repeated five times with thesensors being immersed into the fermenter and with no adjustments madeto readings of each sensor after a sterilisation cycle. The fermenter issampled periodically for pH measurements using a RADIOMETER laboratorymeter. Test results are summarised in Table 3.

TABLE 3 pH readings, 465 Sterili- claimed model SK2403S Differential pHreadings No. sations sensor sensor bench device (4)-(5) (3)-(5) (3)-(4)1 2 3 4 5 6 7 8 1 1 7.80 7.95 7.70 0.25 0.10 −0.15 2 2 6.68 6.92 6.780.14 −0.07  −0.24 3 3 7.04 7.33 6.96 −0.37  −0.08  −0.29 4 4 7.01 7.437.01 0.42 0.00 −0.42 5 5 6.79 7.24 6.69 0.56 0.10 −0.55

It can be seen from these examples that the claimed sensor maintains itscalibration performance after multiple sterilisations and provides agood readings correlation with results obtained by the RADIOMETERlaboratory electrode. The Ingold's pH readings are commonly deviatingfollowing each sterilisation cycle, thus leading to errors in excess of0.5 pH when its readings are not adjusted. Therefore, the claimed sensorhas been reported to have more consistent readings after multiplesterilisations; it maintains operability in any spatial position an isfree from reinstallations while additional adjustments using buffersolutions, thus yielding significant maintenance advantages.

Replacement indicator and reference electrodes offer simple replacementsduring functional changes such as its customisation for activitymeasurements on individual ions or redox potential, or in case ofelectrode (indicator or reference) failures. Solid electrolytes in theindicator electrodes and the specified electrolyte for referenceelectrodes will maintain normal electrode operation in any position,under elevated temperatures and pressures as well as at temperaturesbelow −0° C. and zero-gravity conditions.

Industrial Applications

The following industrial applications are possible for the presentinvention: biotechnology, for monitoring pH and redox potential infermenters; chemical and pharmaceutical industries, pH and redoxpotential monitoring for chemical synthesis of derivatives used forproduction of antibiotic recipes such as Chloramphenicol and othersynthetic preparations; heat and power engineering, for water treatmentunit monitoring at thermal power plants; geology and environmentalmonitoring, for continuous, high-depth pH, redox potential andgroundwater composition measurements.

The invention claimed is:
 1. A multipurpose ion-selective sensorcomprising: a hollow housing tube comprising a first connecting memberand a second connecting member, fixed in an alignment, said connectingmembers being separable from each other; an indicator electrodeelectrically connected with a measurement transmitter, whereby theindicator electrode has a solid indicator system comprising a solidsensing membrane to be immersed into the analyzed liquid; a referenceelectrode comprising a first hollow body and a second hollow bodywherein the first hollow body is located in the second hollow body; thefirst hollow body containing: a potential-forming semi-element locatedin the first hollow body, the semi-element is adapted to be electricallyconnected to the measurement transmitter, a first connecting elementlocated in a surface of the first hollow body; and a first electrolytefilled with a first solid dispersed material forming a first spatialstructure in the first electrolyte, located in the first hollow body,wherein the first spatial structure is rigidly linked with an internalsurface of the first hollow body, the potential-forming semi-element andthe first connecting element; the second hollow body containing: asecond connecting element located in a surface of the second hollowbody, for contacting an analyzed liquid; and a second electrolyte filledwith a second solid dispersed material which forms a second spatialstructure, located in the first hollow body, wherein the second spatialstructure is rigidly linked with an internal surface of the secondhollow body, an external surface of the first hollow body and the secondconnecting element; wherein the concentration of the first soliddispersed material in the first electrolyte is lower than theconcentration of the second solid dispersed material in the secondelectrolyte; and wherein the reference electrode is rigidly fixed on thefirst connecting member and at least one indicator electrode is rigidlyfixed on the second connecting member.
 2. The multipurpose ion-selectivesensor as set forth in claim 1, wherein the first solid dispersedmaterial of the first electrolyte is present in the in the first hollowbody in the quantity sufficient for its concentration to be between 0.1and 5 wt. % and the second solid dispersed material of the secondelectrolyte is present in the second hollow body in the quantitysufficient for its concentration to be between 5 and 30 wt. %, wherebyboth solid dispersed materials contain acrylamide monomers andNN′-methylenebisacrylamide in proportion of 5 to 35 weight parts ofacrylamide per one weight part of NN′-methylenebisacrylamide and amonomer polymerization initiator.
 3. The multipurpose ion-selectivesensor as set forth in claim 1, comprising a resistance temperaturedetector, which is electrically connected to the measurementtransmitter.
 4. The multipurpose ion-selective sensor as set forth inclaim 3, wherein the resistance temperature detector is located outsidethe sensor in proximity of the electrode location.
 5. The multipurposeion selective sensor as set forth in claim 3, wherein the resistancetemperature detector is mounted inside the sensor.
 6. The multipurposeion-selective sensor as set forth in claim 1, comprising the hollowhousing tube with a holder for fixing it to a container containing theanalyzed liquid zone.
 7. A multipurpose ion-selective sensor comprising:a hollow housing tube; an indicator electrode located in the hollowhousing tube, the indicator electrode is adapted to be electricallyconnected to a measurement transmitter; and a reference electrodecomprising: a second hollow body; a first hollow body located in thesecond hollow body; the first hollow body containing: apotential-forming semi-element located in the first hollow body, thesemi-element is adapted to be electrically connected to the measurementtransmitter, a first connecting element located in a surface of thefirst hollow body; and a first electrolyte filled with a first soliddispersed material forming a first spatial structure in the firstelectrolyte, located in the first hollow body, wherein the first spatialstructure is rigidly linked with an internal surface of the first hollowbody, the potential-forming semi-element and the first connectingelement; the second hollow body containing: a second connecting elementlocated in a surface of the second hollow body, for contacting ananalyzed liquid; and a second electrolyte filled with a second soliddispersed material which forms a second spatial structure, located inthe first hollow body, wherein the second spatial structure is rigidlylinked with an internal surface of the second hollow body, an externalsurface of the first hollow body and the second connecting element; andwherein the concentration of the first solid dispersed material in thefirst electrolyte is lower than the concentration of the second soliddispersed material in the second electrolyte.
 8. The multipurposeion-selective sensor of claim 7, wherein: the concentration first soliddispersed material in the first electrolyte is between 0.1 and 5 wt. %;and the concentration of the second solid dispersed material in thesecond electrolyte is between 5 and 30 wt. %.
 9. The multipurposeion-selective sensor of claim 8, wherein: the reference electrode andthe indicator electrode are detachable from the hollow housing tube; andthe indicator electrode comprises a solid sensing membrane to beimmersed into the analyzed liquid, a solid electrolyte on an interiorsurface of the solid sensing membrane and a conductor in contact withthe solid electrolyte and the measurement transmitter.